1. Field of the Invention
The present invention relates to delay lines, and more particularly, to a delay line that is usable for delaying signal transfer in a computer or a measuring instrument.
2. Description of the Related Art
FIG. 8 and FIG. 9 are respectively a top view of a conventional stripline-type delay line 50 and a cross-section thereof taken along the direction of arrow IX--IX. In the delay line 50, a transmission line 52 used for carrying a signal is folded into a meandering shape and is embedded in a dielectric 51. Ground conductors 53 and 54 (see FIG. 9) are formed on both main surfaces of the dielectric 51 such that they are separated by the transmission line 52 and by dielectric layers 51a and 51b (see FIG. 9) constituting the dielectric 51.
The ground conductors 53 and 54 are connected to ground terminals 55 and 56 (see FIG. 8), respectively, the ends of the transmission line 52 are connected to an input terminal 57 and an output terminal 58 (see FIG. 8), respectively, and an external resin 59 covers the surrounding area. In the figures, D (see FIG. 9) indicates the distance between the ground conductors, and G (see FIG. 8) indicates the distance between opposing line sections of the transmission line.
The delay time td of the delay line 50 is generally expressed by the following expression, where 1 indicates the total length of the transmission line 52, Co indicates the speed of light, and .di-elect cons.r indicates the relative dielectric constant of the dielectric layers 51a and 51b: ##EQU2##
In other words, by adjusting the total length l of the transmission line 52, the delay time td can be set to any value.
The conventional delay line described above has the problem that its delay-time varies greatly as a function of frequency. FIG. 10 shows the frequency characteristics of a conventional delay line in which the delay time is designed to be 1.0 ns. A frequency range wherein the delay time is 1.0 ns.+-.50 ps (design value.+-.5%) ranges only from 180 MHz to 330 MHz. Recently, however, the need has arisen for electronic units to be usable at higher frequencies, and delay lines are needed to be used in various frequency ranges. However, this conventional delay line is difficult to use over a wide frequency range.
To solve this problem, namely, to keep the delay time within the design value.+-.5%, G/D has been set to 0.6 or more, since G/D has been found to have a relationship with the variation in the delay time, as shown in FIG. 11. To set G/D to 0.6 or more, however, one of the following measures must be taken:
1. The distance G between opposing line sections of the transmission line must be enlarged; or PA1 2. The distance D between the ground conductors must be reduced
If measure (1) is selected, the width of the delay line is enlarged to increase the area, so it is difficult to obtain a compact delay line.
If measure (2) is selected, the dielectric layers provided between the transmission line and the ground conductors are made thinner, so it is difficult to control the thickness of the dielectric layer. In addition, the line width is made narrower in order to achieve impedance matching, so it is difficult to manufacture the delay line.